1. Field of the Invention
The present invention relates to a millimeter-band switching circuit which operates in a millimeter-band.
2. Description of the Related Art
In a general millimeter-band switching circuit, switching elements are connected in parallel with a signal line to reduce an insertion loss. In order to ensure isolation, two or more switching elements are connected with one another through a transmission line.
A known millimeter-band single-pole single-throw (SPST) switching circuit will be described with reference to FIGS. 7 and 8. FIG. 7 is an equivalent circuit diagram showing a structure of the known millimeter-band SPST switching circuit (see, for example, JP 09-093001 A). FIG. 8 shows an insertion loss and an isolation characteristic of the known millimeter-band SPST switching circuit. The characteristics shown in FIG. 8 are obtained by simulation.
The known millimeter-band SPST switching circuit shown in FIG. 7 includes a transmission line L2 connected with a signal input and output terminal P1, a transmission line L2 connected with a signal input and output terminal P2, a transmission line L4 connected between the two transmission lines L2, a field effect transistor (FET) T whose gate is connected with a control voltage application terminal V1 through a bias resistor R, whose drain is connected with the transmission lines L2 and L4, and whose source is grounded, and a field effect transistor (FET) T whose gate is connected with the control voltage application terminal V1 through a bias resistor R, whose drain is connected with the transmission lines L2 and L4, and whose source is grounded. Note that the transmission lines expressed by the same reference symbol have the same characteristic impedance.
When an FET parameter is changed in the known millimeter-band SPST switching circuit, it is necessary to redesign a line length of the transmission line L4 located between the two FETs. FIG. 8 shows a characteristic variation example in a case of a 77 GHz-band switching circuit. In FIG. 8, Loss (dotted line) indicates an insertion loss before variation and ISO (dotted line) indicates an isolation characteristic before variation. When an off capacitance of an FET is increased by, for example, 20%, the insertion loss becomes Loss′ (alternate long and short dash line) shown in FIG. 8 and the isolation becomes ISO′ (alternate long and short dash line) shown therein. Therefore, there is a problem that the characteristics are shifted to a low-frequency side. In order to solve such a problem, it is necessary to shorten the line length of the transmission line L4, thereby shifting the positions of the FETs.
A known millimeter-band single-pole double-throw (SPDT) switching circuit will be described with reference to FIG. 9 (see, for example, JP 2003-179402 A). FIG. 9 is an equivalent circuit diagram showing a structure of the known millimeter-band SPDT switching circuit.
The known millimeter-band SPDT switching circuit shown in FIG. 9 includes a transmission line L2 connected with a signal input and output terminal P1, a transmission line L2 connected with a signal input and output terminal P2, an inductor LL connected between the input and output terminal P1 and a control voltage application terminal V1, an inductor LL connected between the input and output terminal P2 and a control voltage application terminal V2, a transmission line L1′ connected with one of the transmission lines L2, a transmission line L1′ connected with the other of the transmission lines L2, two capacitors C1, each of which is connected between corresponding one of the transmission lines L1′ and a branch point, and a transmission line L5 connected between a signal input and output terminal P0 and the branch point. The switching circuit further includes a first diode D whose anode is connected with the transmission lines L2 and L1′ and cathode is grounded, a second diode D whose anode is connected with transmission line L1′ and a capacitor C1 and cathode is grounded, a third diode D whose anode is connected with the transmission lines L2 and L1′ and cathode is grounded, and a fourth diode D whose anode is connected with transmission line L1′ and a capacitor C1 and cathode is grounded.
When the diodes D are used as the switching elements, a control voltage is applied thereto through the inductor LL for cutting off an RF signal. However, it is necessary to provide the capacitor C1 for cutting off the voltage in the branch point. Therefore, there is a problem that the insertion loss is increased by the capacitor C1.
A millimeter-band switching circuit in which a coupling line is provided between two switching elements has been proposed (see, for example, JP2003-224404 A). In the conventional millimeter-band switching circuit, two structures in each of which the coupling line is provided between the switching elements are symmetrically arranged and a capacitor or an inductor is connected between the structures.
As described above, in the conventional millimeter-band SPST switching circuit, when the off capacitance of the FET (switching element) increases, there is a problem that the insertion loss and the isolation characteristic are shifted to the low-frequency side. In order to solve such a problem, it is necessary to shorten the line length of the transmission line L4 between the two FETs, which leads to another problem in that the FETs need to be shifted in position.
As described above, in the known millimeter-band SPDT switching circuit, when the diodes D are used as the switching elements, it is necessary to provide the capacitor C1 for cutting off the voltage in the branch point. Therefore, there is a problem that the insertion loss is increased by the capacitor C1.
The conventional millimeter-band switching circuit using the coupling line has a problem that the number of parts is large, thereby increasing cost.